Auto merge of rust-lang#107055 - kylematsuda:eb-fn-sig, r=lcnr

Switch to `EarlyBinder` for `fn_sig` query

Part of the work to finish rust-lang#105779 (also see rust-lang/types-team#78).

Several queries `X` have a `bound_X` variant that wraps the output in [`EarlyBinder`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/struct.EarlyBinder.html). This adds `EarlyBinder` to the return type of the `fn_sig` query and removes `bound_fn_sig`.

r? `@lcnr`

src/main_shim.rs

@@ -46,7 +46,7 @@ pub(crate) fn maybe_create_entry_wrapper(
         is_main_fn: bool,
         sigpipe: u8,
     ) {
-        let main_ret_ty = tcx.fn_sig(rust_main_def_id).output();
+        let main_ret_ty = tcx.fn_sig(rust_main_def_id).no_bound_vars().unwrap().output();
         // Given that `main()` has no arguments,
         // then its return type cannot have
         // late-bound regions, since late-bound